Ink jet printing involves ejecting ink droplets from orifices in a print head onto a receiving surface to form an image. The image is made up of a grid-like pattern of potential drop locations, commonly referred to as pixels. The resolution of the image is expressed by the number of ink drops or dots per inch (dpi), with common resolutions being 300 dpi and 600 dpi.
Ink-jet printing systems commonly utilize either direct printing or offset printing architecture. In a typical direct printing system, ink is ejected from jets in the print head directly onto the final receiving substrate. In an offset printing system, the image is formed on an intermediate transfer surface and subsequently transferred to the final receiving substrate. The intermediate transfer surface may take the form of a liquid layer that is applied to a support surface, such as a drum. The print head jets the ink onto the intermediate transfer surface to form an ink image thereon. Once the ink image has been fully deposited, the final receiving substrate is then brought into contact with the intermediate transfer surface and the ink image is transferred to the final receiving substrate.
U.S. Pat. No. 5,389,958 entitled IMAGING PROCESS and assigned to the assignee of the present application (the '958 patent) is an example of an indirect or offset printing architecture that utilizes phase change ink. The intermediate transfer surface is applied by a wicking pad that is housed within an applicator apparatus. Prior to imaging, the applicator is raised into contact with the rotating drum to apply or replenish the liquid intermediate transfer surface.
Once the liquid intermediate transfer surface has been applied, the applicator is retracted and the print head ejects drops of ink to form the ink image on the liquid intermediate transfer surface. The ink is applied in molten form, having been melted from its solid state form. The ink image solidifies on the liquid intermediate transfer surface by cooling to a malleable solid intermediate state as the drum continues to rotate. When the imaging has been completed, a transfer roller is moved into contact with the drum to form a pressurized transfer nip between the roller and the curved surface of the intermediate transfer surface/drum. A final receiving substrate, such as a sheet of media, is then fed into the transfer nip and the ink image is transferred to the final receiving substrate.
To provide acceptable image transfer and final image quality, an appropriate combination of pressure and temperature must be applied to the ink image on the final receiving substrate. U.S. Pat. No. 5,777,650 entitled PRESSURE ROLLER and assigned to the assignee of the present application (the '650 patent) discloses a roller for fixing an ink image on a final receiving substrate. The preferred embodiment of the roller is described in the context of an offset ink jet printing apparatus similar to the one described in the '958 patent. In this embodiment, the final receiving medium is preheated to a preferred temperature of about 63.degree. C. and the pressure in the transfer nip is preferably about 1150 psi (7,929 kPa). Additionally, the speed of the final receiving medium through the transfer nip is approximately five inches/sec. (13 cm./sec.).
In a color printing system, the ink image on the final receiving surface is composed of individual drops of ink that form primary and secondary colors. The primary and/or secondary colors may include two or more drops of ink placed on top of one another. In the image transfer process, the ink image is transferred from the drum to the final receiving substrate. A portion of the ink image is fused or pressed into the final receiving substrate. The height of the remaining ink that lays above the surface of the final receiving substrate is referred to as the "ink pile height."
The ink pile height of an image affects the "look and feel" of the image. In general, a lower ink pile height is preferred, as the appearance of the image will more closely resemble an image created by a commercial web press. The ink pile height also affects the ability of a user to write on the image. In images having ink pile heights approaching 1.times.10.sup.-3 in., and higher, the tip of a writing instrument will often furrow through the ink "pile." This can hinder the flow of writing ink through a ball point pen, or prevent the graphite writing surface of a pencil from contacting and marking the receiving substrate. Additionally, depending upon the composition of the ink used in the printer, ink pile height can hinder media from being transported through an automatic document feeder in a photocopier.
In the prior art offset phase change ink printers, such as the printer described in the '958 patent, the ink pile height of images on the final receiving surface ranges from about 1.times.10.sup.-5 inch for a single pixel primary color to about 1.times.10.sup.-3 in. for a solid fill secondary color. By comparison, a liquid ink jet printer using a direct printing process and an aqueous-based ink produces images having a negligible ink pile height of less than 1.times.10.sup.-5 inch.
In the image transfer process described above for the '958 patent, higher temperatures and pressures in the transfer process will generally yield lower ink pile heights. However, higher pressures in the transfer process also increase the loadings on the pressure roller, support surface or drum and other printer components. This accelerates wear on these components and tends to limit the maximum printing speed of the apparatus. Increased nip temperatures can inhibit duplex printing and cause the ink image to partially liquify and smear. These undesirable effects are magnified in an offset printing system in which the image transfer process is performed continuously; that is, the support surface or drum is under continuous loading and a high nip temperature is maintained. Thus, a need remains for an image fusing system that reduces ink image pile height, allows faster print speeds, reduces the transfer nip pressure and overcomes the other drawbacks of the prior art.